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fable 1. — Minerals Which Occur in Meteorites 

 Chemical elements which form theni 



Name of 

 mineral 



Stone-iron meteorites, such as the one illustrated above, have a distinc- 

 tive appearance unlike any naturally-occurring terrestrial rock. 



who was a leading meteorite worker and was a curator at 

 Field Museum from 1894 to 1933. 



At the present time meteoritics has become quite cosmo- 

 politan. People who work in it include physicists, chemists, 

 geologists, astronomers, metallurgists, organic chemists, en- 

 gineers, and statisticians. In general, this study can be di- 

 vided up into four main categories: (1) mineralogy and 

 chemical composition of meteorites; (2) ballistics of meteo- 

 rites — the study of meteorite orbits, fall phenomena, and 

 impacts; (3) physics of meteorites — age determinations, 

 magnetic features, radioactivities, cosmic ray effects; (4) 

 organic chemistry- — study of organic compounds in some 

 meteorites. 



Although it is convenient to divide up these areas of 

 study it must be understood that each one interacts with the 

 other so it is hard to talk about any one category exclusively. 

 Nevertheless, for the remainder of this article I will deal 

 mostly with category (1). 



Because of the ancient practice of worshipping meteo- 

 rites, it has come down to us today that they are something 

 unique and special. In reality, most of them are very much 

 like some terrestrial rocks, and some few of them are quite 

 difficult to distinguish visually from certain kinds of com- 

 mon earth rocks. Of the seven common minerals which 

 make up most meteorites (see Table 1) only one of them, 

 schreibersite, is not known to occur in terrestrial rocks, but 

 it is found in some man-made steel mill products, and could 

 occvir naturally at depth within the earth. 



1. Olivine Magnesium, iron, silicon, oxygen 



2. Pyroxene Magnesium, iron, calcium, silicon, oxygen 



3. Feldspar Sodium, calcium, potassium, aluminum, 



silicon, oxygen 



4. Metal Iron, nickel, cobalt 



5. Troilitc Iron, sulfur 



6. Graphite Carbon 



7. Schreibersite Iron, nickel, phosphorus 



Table 2. — Meteorite Groups 

 Group Subgroups Principal minerals making up each group 



STONE Chondrites Olivine, pyroxene, metal, troilite, feldspar 



.^chondrites Feldspar, pyroxene 

 STONE-IRON Pallasites Metal, olivine 



Mesosiderites Pyroxene, metal 

 IRON Octahedrites Metal, troilitc, graphite, schreibersite 



(has Widmanstatten pattern) 

 Ataxites Metal, schreibersite, troilite, graphite 



(has no Widmanstatten pattern) 



Meteorites fall fairly neatly into three groups depending 

 on which combinations of these minerals make them up 

 (see Table 2). Because they are generally composed of the 

 same minerals as some terrestrial rocks, the question always 

 arises, how are they to be identified? For iron meteorites this 

 problem is not so difficult as for a stone meteorite. Although 

 iron and iron-nickel metal does occur in terrestrial rocks, it 

 is extremely rare, being limited to a few occurrences in Ore- 

 gon, New Zealand, Germany, and Greenland. So, except 

 for the added complication of man-made steel mill scrap and 

 slag, which can be found almost anywhere in industrialized 

 nations like the United States, pieces of metal which are 

 found stand a good chance that they are not natural terres- 

 trial irons. In addition, the Austrian scientist, A. von VV^id- 

 manstatten, in 1808 discovered that the metal in most iron 

 meteorites forms in two diflferent kinds of structures in a 

 regular geometrical arrangement. The reasons for this are 

 now very well understood but are beyond the scope of this 

 particular article. What is important here is that by cutting 

 flat, polishing, and slowly etching with acid, this geometrical 

 pattern of iron-nickel can be brought out, much like chem- 

 ical treatment brings out the image on an exposed piece of 

 film. This kind of structure is unique and is not found in 

 any terrestrial iron, or man-made metal product. These 

 iron meteorites are called octahedrites and the geometrical 

 pattern is called a Widmanstatten pattern in honor of the 

 discoverer. Among the iron meteorites only 14% of them 

 have compositions which do not consist of two kinds of 

 metal structure, but only one. For these no geometrical 

 etch pattern exists and other tests must be used. 



Stone meteorites are not quite so easy. Fortunately, 

 however, most stone meteorites fall into a major subgroup 

 called chondrites [con'-drites], because they contain small 

 spherical clusters of grains of the minerals olivine, pyroxene, 

 and feldspar, called chondrules [con'-drools]. The chon- 

 drules are held together by a matrix of fine grains of the 

 same minerals plus some metal and troilite. These little 

 spheres range from less than a tenth of an inch to occasional 



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